I used the built in Android feature to save a website as an app (Zcal supports being made into a PWA) and it works totally fine as a mobile app. Is there additional functionality you would want?

with Android/Chrome, you can install zcal as an app. I've noticed with Discourse, when you load the site on Android, it suggests you install as app -- this is a nice bit of polish. I find this actually more convenient than an app-store app.